Transistor amplifier circuit



Nov. 5, 1957 A. J. w. M. VAN OVERBEEK 281239 TRANSISTOR AMPLIFIER CIRCUIT Filed Aug. 21,

INVENTOR ADRIANUS JOHANNESW!LHELNIUS MARIE VAN OVERBEEK AGENT United States Patent 2812,390 TRANSISTR AMPLIFILER cmcurr Adrianus Johannes Wilhelmus Mario van Overbeek, Eindhoven, Netherlands, assignor, by mesne assignments, to

North American Philips Company, Inc., New York,

NY, a corporation of Delaware Application August 21, 1953, Serial No. 375,715

Claims priority, application Netlierlnds September 27, 1952 1 Claim. (Cl. 179--171) 2,812390 Patented Nov. 5, 1957 2 e al al V\ where Vc signifies the voltage across the collector elec trede and or the current amplification factor of the first transistor.

is connected to the input oremitter electrode of the next subsequent transistor through a series resonant'circuit tuned to the input frequency so thatthe collector current of the firstmentioned transistor flows substantially cntirely to the emitter electrode of the second transistor. The contribution of the first transistor towards the total amplification of the circuit is thus substantially deternijined by the current amplification factor Ot of said first transistor.

The invention permits obtaining a materially larger amplification by means of such a circuit. Accordng to the invention, the impedance between the collector electrode of a preceding transistor of the cascade and the emitter electrode of the next subsequent transistor behavcs, as far as the input frequency is concerned, like an inductance which, as far as the input freqnency is con cerned, is in resonance with the capacitor between the collector and the base electrode of the first-mentioned transistor, which capacitor is constitnted wholly or in part by stray capacities.

The invention will now be described with reference to the accompanying drawing, in which:

Fig. 1 is a schematic diagram of an embodiment of the amplifier circuit of the present invention; and

Fig. 2 is a modification of the embodiment of Fig. 1.

Referring now to Fig. 1, two cascade connected transistors 1 and 2 each comprise an e'mitter electrode e, a collector electrode c and a base electrode b. A source 4 supplying input oscillations required to be amplified is included in the circuit between the emitter electrode e and the base electrode b of the first transistor 1 with the result that sirnilar oscillations are set up across the collector electrode 0 of said transistor 1. These oscillations are supplied through a coupling inductance L and a buler capacitor 5 to the emitter electrode e of the second transistor 2 so that the amplified oscillations are set up across a load impedance 6 included in the circuit between the collector electrode 0 and the base electrode b of the second transistor 2.

The inductance L of the above-mentioned known circuit arrangement is in series resonance with the capacitor 5 for the input frequency of the source 4. Thus the emitter current ie2 of the second transistor 2 becomes substantially equal to the collector current c1 of the first transistor, since the internal input impedance r of the second transistor is negligible as compared with the impedance of the stray collector capacity C of the first transistor. Since the impedance r is also negligible as compared with the internal collector impedance Rc of the first transistor, the amplification of the first transistor is found to be According to the invention, in contradistinction thereto 5 designates a buler capacitor which constitutes a substantially negligible impedance for the signal frequency, and the inductance L together with the stray capacity C of the collector electrode c of the first transistor 1 is tuned to the input frquency. Assnmingthe damping of the resonant circuitLCr tobe substantiallyconcentrated in the internal input resistance r of the secondtransistor 2, the external impedance in the collector circuit of the first transistor is given in the case of resonance by Cr so that on calcnlation the collector voltage is found to be L i =R"L C1R +L 61 where Rm designates the mutual impedance of the transistor.

In this case, the current e2 is found to be 1 VV ywL-lr jwL on the assumption that r wL, where w is the angular frequency and j=\/-l,

Conseqently, if the amplification of the first transistor 1 is calculated, a value is found which is R, R, w(CTR+L) 1 wC' times as great.

Normally in a point contact (A) transistor Re is some dozens of kilohms and in an NPN transistor some dozens of megohms, whereas r is only some hun'cireds of ohms and C some few micrornicrofarads. Assurning now, for example for a point 'contact transistor, that Rc=20 kilohms, r=200 ohms, C=5 micrornicrofarads and w= megacycles per second, a five-fold gain in amplification is found. For an NPN transistor this gain may even be considerably larger.

The highest amplification is fonnd at that value of the input frequency at which 1 w=-- Cw RJ This value is determined by difierentating the preceding equation with respect to w and equating the difierential to zero. At a given input frequency and given values of Rc and r it may therefore be advantageous artificially to increase C by adding a small capacitor.

The circuit shown in Fig. 2 is a modification of the one shown in Fig. 1, the bias voltages of the emitter electrodes e of the transistors 1 and 2 being produced in a known manner by means of bypassed base resistors 9 and 10. Like circuit elements are otherwise designated by the same reference numerals as in Fig. 1. If clesred, feedback rriay also be provided by means of a nombypassed base resistor (not shown) or by intercoupling the collector and the emitter electrode by a capacitance 12. In this case,

hearing in mind that the base resistor or internal transistor resistance betwcen the emitter and the base electrode are small as compared with the irnpedanceof the capacitor C, the total capacity as measured between the collector and the base electrode must be taken as the value of the said capacitor.

The circuit has been described with respect to the case of two cascade-connected transistors. Obviously the cascade may be extended to comprise more transistors which may also be intercoupled in the manneraccording to the invention.

While the inventiou has been described by means of specific examples and in 'specific embodirnents, I do not wish to be limited thereto, -for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What is clarned is:-

An amplifier circuit for electric input oscillations having a given frequency spectrum including a given frequency value, comprising first and second transistors each having an emitter, a base, and a collector electrode and en in ternal input impedance, means for applying said input oscillations to the emitter electrode of said first transistor, and a network interconnecting the collector electrode of said first transistor and the emitter electrode of said second transistor, said network comprising a capacitor shunting the collector and base electrodes of said first transistor, said capacitor having a value substantially determined by the relationship CVR whercin m is equal to 21r tmes the said gven frequency value, C is the capactance of said capacitor, Re is the internal collector impedance of said first transistor and r is the internal input impedance of said second transistor, an inductance interconnecting the collector electrode of said first transistor and the emitter electrode of said sec 0nd transistor and said input mpedance of said second transistor, said input impedance of said second transistor being of substantially low value relatve to the reactance of said nductance, said capacitor and nductance having values producing resonance in said netwerk at the said given frequency value.

References Cited in the file of this patent OTHER REFERENCES Bell Telephone Laboratories, Ine. (Publsher), ,Ihe Transistor, Bell Laboratory Text, published December 4, 1951 (particularly pp. 357 and 375). 

